Line ratios of C¹⁸O/¹³CO (J = 2−1), defined as L₁₈/L₁₃, as function of disk masses for models where i = 10°. The top panels report the results for T Tauri disk models and lower panels those for Herbig disk modelsl. Left, middle and right panels present results for models with respectively R_c = 30 au, R_c = 60 au, and R_c = 200 au. Each point reports the result for the median line ratio in each mass bin. The continuous solid lines show the fit function reported in Eq. (3). The filled regions show the standard deviation from the median values.